Typical submicron integrated circuit fabrication processes include, for example, the definition of isolation oxides, sometimes called field oxides 13 (FIG. 1) upon a substrate 11. The field oxides 13 define a comparatively flat region 15 of the substrate which is often termed "moat". In the initial stages of, illustratively, CMOS fabrication, the moat is covered with a sacrificial grown oxide 17.
After formation of the field oxide 13 and sacrificial oxide 17, one or more ion implantation steps is performed, for example, to define either a p-tub or an n-tub, or to adjust the threshold voltage of the subsequently formed transistors. Illustrative implantation species are: arsenic, phosphorus, boron, or boron fluoride. Some of the implanted species are denoted by reference numeral 19.
The ion implantation step tends to change the stoichiometry of the upper portions of field oxide 13, thereby making the upper portion of field oxide 13 susceptible to being etched more quickly in a wet etch solution than oxides which have not been influenced or damaged by ion implantation.
After dopant species 19 has been implanted into substrate 11, a wet etch (illustratively using HF) is performed to remove sacrificial oxide 17 (so that a gate oxide may be subsequently grown). The wet etch attacks upper surface 21 of field oxide 13, thereby etching the upper surface of field oxide downward to a location approximately indicated by reference line 23.
The accelerated etching of field oxide 13 described above is sometimes called "birds beak pull back". Such "birds beak pull back" undesirably reduces the vertical height of field oxide 13. Such height reduction is particularly undesirable for submicron devices which utilize comparatively thin field oxides. Such thinning of the field oxide adversely affects the effectiveness of the field oxide in inhibiting parasitic transistor action. Furthermore, the birds beak pull back exposes a portion of the substrate which has a different dopant concentration than the majority of the moat area. The above-mentioned problems adversely affect device yield.
Those concerned with the development of integrated circuit technology have sought more reliable methods of device fabrication.